It is known to use nonionic as well as ionic polymers as thickening agents in water-based systems. Among the ionic polymers, both anionic and cationic polymers or polyelectrolytes are known. By a polyelectrolyte is here meant a polymer having a plurality of charged or ionizable groups, an anionic polyelectrolyte containing negatively charged groups and a cationic polyelectrolyte containing positively charged groups. It is also known to combine anionic and cationic polyelectrolytes.
By providing a nonionic or an ionic polymer with hydrophobic groups, these properties can be modified.
As an example of prior-art technique, the following documents can be mentioned.
EP 0 390 240 discloses nonionic cellulose ethers which are usable as thickening agents for water-based paints. The cellulose ethers are modified with pendent groups containing both hydrophobic and hydrophilic elements.
WO 94/06840 discloses a thickening agent for water-based paints, which consists of a water-soluble polyurethane of the so-called comb polymer type. The polyurethane has water-soluble oxyethylene groups and hydrophobic groups having 8-22 carbon atoms coupled to the main polymer chain.
U.S. Pat. No. 4,839,166 relates to a thickening agent resulting from the interaction on the one hand of a copolymer of cellulose or of a cellulose derivative which is grafted with a quaternary ammonium salt of a water-soluble monomer and, on the other hand, of a carboxylic anionic polymer. As a particularly preferred example of the cationic polymer, mention is made of hydroxyethyl cellulose copolymer grafted with diallyldimethylammonium chloride. As especially preferred examples of the anionic polymer, mention is made of copolymers of methacrylic acid with methyl methacrylate, monoethyl maleate, butyl methacrylate or maleic acid. Neither the cationic polymer nor the anionic polymer contains any hydrophobic groups.
DE 33 02 456 discloses the production of a cationic cellulose derivative and its use as conditioning agent in cosmetics in combination with anionic surfactants. The cationic cellulose derivative is obtained by quaternarisation of a cellulose ether containing dialkyl aminoalkyl ether groups, the alkyl parts having 1-4 carbon atoms. No combination of a cationic polymer and an anionic polymer is disclosed, nor a combination of such polymers as contain hydrophobic groups.
EP 0 130 732 relates to a water-soluble composition containing a particular anionic polymer and at least one cationic or amphoteric polymer which is selected among poly(diallyldimethylammonium chloride) and three further defined types of polymers. No modification of the anionic polymer and the cationic polymer with hydrophobic groups is stated. The described composition is intended to be used as thickening agent when treating subterranean formations such as wells.
U.S. Pat. No. 4,970,260 discloses a thickening agent for salt-containing aqueous solutions, which is based on a polymer complex of a water-soluble, anionic polymer having a low charge density and a water-soluble, cationic polymer having a low charge density, one of the polymers being present in an excess of at least about 20 mole % above the other. None of the two ionic polymers contains modifying hydrophobic groups.
U.S. Pat. No. 3,902,958 discloses a liquid dispersion containing a water-soluble anionic vinyl addition polymer and a water-soluble cationic polymer. The dispersion is used as additive in papermaking. Preferred examples of ionic polymers do not comprise polymers which are modified with hydrophobic groups, and no combination of anionic and cationic polymers which are modified with hydrophobic groups is disclosed.
An aqueous solution containing two different polymers normally separates in two phases. The phase separation can either result in both polymers separating in different phases, or it is possible to obtain an increased concentration of both polymers in a phase that is present together with a solvent-rich phase. The first phenomenon is called segregative phase separation and is usually observed in nonionic polymers or polyelectrolytes of the same charge and the corresponding charge density. The second phenomenon is called associative phase separation and occurs in mixtures of polyelectrolytes of opposite charge. Such an associative phase separation often takes place over practically the entire mixing range of both polyelectrolytes.